The dose- and time-dependent induction of P45OIIE1 in rat liver by pyridine has been characterized. A single injection of pyridine (100 mg/kg, i.p.) increased P450IIE1 levels 2-, 3- and 4-fold at 6, 10 and 24 hr, respectively, relative to controls as evidenced by metabolic activity and Western blot analysis. Induction of IIE1 was dose-dependent over the range 10 to 200 mg/kg. Cycloheximide administration completely prevented the induction of P450IIE1 by pyridine, whereas actinomycin D administration had no appreciable effect. Pyridine induction of IIE1 did not occur by transcriptional activation. Hybridization analysis failed to reveal an increase in IIE1 message in either total RNA or poly(A+) mRNA following pyridine treatment, although a slight decrease in poly(A+) mRNA was noted. The rate of P450IIE1 synthesis was assessed by labelling of proteins with [14C]leucine in vivo, followed by autoradiographic analysis. Increased intensity of a protein band comigrating with purified IIE1 was observed in microsomes isolated from rats at 5 hr following either pyridine or acetone treatment, as compared to controls. An increase in intensity was also noted for protein bands migrating in the region of 62 and 50 kDa. These results suggest that induction of P450IIE1 at early times following acute pyridine exposure involves protein synthesis possibly through increased translational efficiency.